1. Field of the Invention
The present invention relates to an endoscope system for acquiring a first image and a second image of different areas in a same subject, and a method for operating the endoscope system.
2. Description of the Related Art
Recently, a wide angle endoscope having a front-view observation optical system and a lateral-view observation optical system has been proposed in order to make it possible to observe a wider range.
Such wide angle endoscopes include a type in which a front-view subject image through a front-view observation window and a lateral-view subject image through a lateral-view observation window are formed on one image pickup device, for example, as described in Japanese Patent No. 4782900, type in which a camera including an optical system and an image pickup device which are combined is provided for each of front-view and lateral-view, as described in Japanese Patent Application Laid-Open Publication No. 2013-542467 (International Publication No. 2012/056453), and others.
Such a wide angle endoscope having the front-view observation optical system and the lateral-view observation optical system is provided with an illumination portion configured to radiate light forward and an illumination portion configured to radiate light laterally, separately. Furthermore, some wide angle endoscopes can also adjust an amount of light radiated forward and an amount of light radiated laterally, separately.
By the way, in a case of radiating illuminating light to a subject, a state occurs in which illuminance is high in a proximal part and low in a remote part, and the proximal part and the remote part are observed brighter and darker, respectively. More specifically, in a case of observing, for example, a luminal subject using a wide angle endoscope, it may happen that a central part of an observation site by front-view becomes a remote part in an axial direction of a lumen and becomes dark, and an observation site by lateral-view becomes a proximal part of a luminal inner wall and becomes bright. Furthermore, in a case where the endoscope comes close to a left side wall in the lumen, for example, a subject distance differs on left and right sides, and, therefore, such an image is obtained that not only a left lateral field of view and a right lateral field of view become bright and dark, respectively, but also a left side and right side of a forward field of view become bright and dark, respectively.
When brightness of an image becomes imbalanced as described above, an endoscope capable of adjusting an amount of illuminating light for each field-of-view direction as described above, performs amount-of-light control for avoiding brightness imbalance, for example, by increasing an amount of light radiated in a right side direction.
For example, International Publication No. 2011/055613 describes an endoscope system in which brightness of a front-view field-of-view image and brightness of a lateral-view field-of-view image are separately detected, and, based on a detection result, a light source apparatus is controlled so that one of the field-of-view images reaches a brightness target value suitable for observation.
Further, Japanese Patent Application Laid-Open Publication No. 2003-179785 describes a technique in which, in an image photographing apparatus for performing photographing of a wide angle image and photographing of an enlarged image, an amplification factor of each image signal is set so that the wide angle image and the enlarged image are displayed with same brightness.